Monoethylene glycol is used as a raw material in the manufacture of polyester fibres, polyethylene terephthalate (PET) plastics and resins. It is also incorporated into automobile antifreeze liquids.
Monoethylene glycol is typically prepared from ethylene oxide, which is in turn prepared from ethylene. Ethylene and oxygen are passed over a silver oxide catalyst, typically at pressures of 10-30 bar and temperatures of 200-300° C., producing a product stream comprising ethylene oxide, carbon dioxide, ethylene, oxygen and water. The amount of ethylene oxide in the product stream is usually between about 0.5 and 10 weight percent. The product stream is supplied to an ethylene oxide absorber and the ethylene oxide is absorbed by a recirculating solvent stream containing mostly water. The ethylene oxide-depleted stream is partially or entirely supplied to a carbon dioxide absorption column wherein the carbon dioxide is at least partially absorbed by a recirculating absorbent stream. Gases that are not absorbed by the recirculating absorbent stream are recombined with any gases bypassing the carbon dioxide absorption column and are recycled to the ethylene oxide reactor.
The solvent stream leaving the ethylene oxide absorber is referred to as fat absorbent. The fat absorbent is supplied to an ethylene oxide stripper, wherein ethylene oxide is removed from the fat absorbent as a vapour stream. The ethylene oxide-depleted solvent stream is referred to as lean absorbent and is recirculated to the ethylene oxide absorber to absorb further ethylene oxide.
The ethylene oxide obtained from the ethylene oxide stripper can be purified for storage and sale or can be further reacted to provide ethylene glycol. In one well-known process, ethylene oxide is reacted with a large excess of water in a non-catalytic process. This reaction typically produces a glycol product stream consisting of almost 90 weight percent monoethylene glycol, the remainder being predominantly diethylene glycol, some triethylene glycol and a small amount of higher homologues. In another well-known process, ethylene oxide is catalytically reacted with carbon dioxide to produce ethylene carbonate. The ethylene carbonate is subsequently hydrolysed to provide ethylene glycol. Reaction via ethylene carbonate significantly improves the selectivity of ethylene oxide conversion to monoethylene glycol.
The lean absorbent that is supplied to the ethylene oxide absorber is typically aqueous, but in the process disclosed in EP 24 628 the lean absorbent is ethylene carbonate. The fat absorbent, containing ethylene oxide and carbon dioxide dissolved in ethylene carbonate, is sent to a stripper wherein ethylene oxide and carbon dioxide are stripped, and ethylene carbonate is returned as lean absorbent to the ethylene oxide absorber. The stripped ethylene oxide and carbon dioxide are supplied to an ethylene carbonate reactor and react to ethylene carbonate in the presence of an anion-exchange resin, functioning as a carboxylation catalyst.
EP 776 890 discloses a similar process. The lean absorbent that is supplied to the ethylene oxide absorber mainly contains ethylene carbonate and ethylene glycol. The fat absorbent, containing ethylene oxide and carbon dioxide dissolved in ethylene carbonate and ethylene glycol, is supplied directly to an ethylene carbonate reactor wherein ethylene oxide and carbon dioxide react in the presence of a catalyst. The absorption apparatus is operated at low temperature and carboxylation occurs in a subsequent reactor wherein the conditions promote carboxylation.
GB 2 107 712 discloses an alternative process where the gases from an ethylene oxide reactor are supplied directly to a reactor wherein ethylene oxide is converted to ethylene carbonate in the presence of a carboxylation catalyst.
The present inventors have sought to further improve the manufacture of alkylene glycol from an alkene and in particular have sought to provide a process that reduces the complexity (and reduces the cost) of the plant whilst ensuring high selectivity.